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Geothermal Energy Chapter 4 4.5 Environmental and social impacts11 In general, negative environmental impacts associated with geothermal energy utilization are minor. Hot fluid production can emit varying quan- tities of GHGs, which are usually small. These originate from naturally sourced CO2 fluxes that would eventually be released into the atmo- sphere through natural surface venting. The exploitation of geothermal energy does not ultimately create any additional CO2 from the subsur- face, since there is no combustion process, though the rate of natural emissions can be altered by geothermal production depending on the plant configuration. Water is not a limiting factor for geothermal power generation, since geothermal fluids are usually brines (i.e., not competing with other uses). Flash power plants do not consume potable water for cooling and yield condensed water that can, with proper treatment, be used for agricultural and industrial purposes. Binary power plants can minimize their water use with air cooling. Potential adverse effects from disposal of geothermal fluids and gases, induced seismicity and ground subsidence can be minimized by sound practices. Good practice can also optimize water and land use, improve long-term sustainability of production and protect natural thermal fea- tures that are valued by the community. The following sections address these issues in more detail. 4.5.1 Direct greenhouse gas emissions The main GHG emitted by geothermal operations is CO2. Geothermal fluids contain minerals leached from the reservoir rock and variable quantities of gas, mainly CO2 and a smaller amount of hydrogen sul- phide. The gas composition and quantity depend on the geological conditions encountered in the different fields. Depending on technol- ogy, most of the mineral content of the fluid and some of the gases are re-injected back into the reservoir. The gases are often extracted from a steam turbine condenser or two-phase heat exchanger and released through a cooling tower. CO2, on average, constitutes 90% of these non-condensable gases (Bertani and Thain, 2002). A field survey of geothermal power plants operating in 2001 found a wide spread in the direct CO2 emission rates. The average weighted by generation was 122 g CO2/kWh, with values ranging from 4 to 740 g CO2/kWh (Bertani and Thain, 2002). In closed-loop binary-cycle power plants, where the extracted geothermal fluid is passed through a heat exchanger and then completely injected, the operational CO2 emission is near zero. In direct heating applications, emissions of CO2 are also typically neg- ligible (Fridleifsson et al., 2008). For instance, in Reykjavik, Iceland, the CO2 content of thermal groundwater used for district heating (0.05 mg CO2/kWhth) is lower than that of the cold groundwater. In China (Beijing, 11 A comprehensive assessment of social and environmental impacts of all RE sources covered in this report can be found in Chapter 9. Tianjin and Xianyang) it is less than 1 g CO2/kWhth. In places such as Iceland, co-produced CO2, when sufficiently pure, may also be used in greenhouses to improve plant growth, or extracted for use in carbon- ated beverages. In the case of Iceland, the replacement of fossil fuel with geothermal heating has avoided the emission of approximately 2 Mt of CO2 annually and significantly reduced air pollution (Fridleifsson et al., 2008). Other examples of the environmental benefits of geother- mal direct use are at Galanta in Slovakia (Fridleifsson et al., 2008), the Pannonian Basin in Hungary (Arpasi, 2005), and the Paris Basin (Laplaige et al., 2005). EGS power plants are likely to be designed as liquid-phase closed-loop circulation systems, with zero direct emissions, although, if gas separa- tion occurs within the circulation loop, some gas extraction and emission is likely. If the current trend towards more use of lower-temperature resources and binary plants continues, there will be a reduction in aver- age emissions. 4.5.2 Lifecycle assessment Life-cycle assessment (LCA) analyzes the whole lifecycle of a product ‘from cradle to grave’. For geothermal power plants, all GHG emissions directly and indirectly related to the construction, operation and decom- missioning of the plant are considered in LCA. Figure 4.6 shows the result of a comprehensive literature review of geo- thermal electricity generation LCA studies published since 1980, which were screened for quality and completeness (see Annex II for details on methodology). All estimates of lifecycle GHG emissions are less than 50 90 80 70 60 50 40 30 20 10 0 Estimates: References: Flashed Steam EGS 4 4 Lifecycle GHG Emissions of Geothermal Power Generation 3 3 Figure 4.6 | Estimates of lifecycle GHG emissions from geothermal power generation (flashed steam and EGS technologies). Unmodified literature values, after quality screen. (See Annex II and Section 9.3.4.1 for details of literature search and citations.) 418 Lifecycle GHG Emissions [g CO2 eq/kWh]PDF Image | Geothermal Energy 4
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